Structure and classification of titanium alloys
Knowledge of titanium alloy
Titanium is an important structural metal developed in the 1950s. Titanium alloys are widely used in various fields due to their high specific strength, corrosion resistance and heat resistance. Many countries in the world have realized the importance of titanium alloy material, and developed it successively, and obtained practical application. Titanium is the first Ⅳ class B elements in the periodic table, appearance is like steel, melting point 672 ℃, 1 genus refractory metal. Titanium content in the crust is rich, much higher than that of Cu, Zn, Sn, Pb and other common metals. Titanium alloy can be divided into heat resistant alloy, high strength alloy, corrosion resistant alloy (ti-mo, ti-pd alloy, etc.), low temperature alloy and special function alloy (ti-fe hydrogen storage material and ti-ni memory alloy).
Titanium alloy element
Titanium alloy is an alloy based on titanium and other elements. Titanium has two kinds of homogenous crystal: dense hexagonal structure of alpha titanium below 882℃, above 882℃ for the body center cubic titanium. The alloying elements can be divided into three types according to their effects on the phase transition temperature: (1) the elements that stabilize the alpha phase and increase the phase transition temperature are the alpha stable elements, such as aluminum, carbon, oxygen and nitrogen. Among them, aluminum is the main alloy element in titanium alloy, which has obvious effects on increasing the strength at room temperature and high temperature, reducing specific gravity and increasing elastic modulus of the alloy. (2) elements that stabilize beta phase and reduce phase transition temperature are beta stable elements, which can be divided into two types: eutectoid and eutectic. The former has molybdenum, niobium, vanadium, etc. The latter has chromium, manganese, copper, iron, silicon to wait. (3) the elements that have little influence on the phase transition temperature are neutral elements, such as zirconium and tin.
Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloy. Oxygen and nitrogen have a greater solubility in alpha phase, which has a significant strengthening effect on titanium alloy, but reduces plasticity. Normally, the content of oxygen and nitrogen in titanium is below 0.15 ~ 0.2% and 0.04 ~ 0.05%, respectively. Hydrogen solubility in the alpha phase is very small, titanium alloy dissolved too much hydrogen will produce hydride, making the alloy brittle. Hydrogen content in titanium alloys is usually controlled at less than 0.015%. Hydrogen in titanium is reversible,
Titanium is an isomer with a melting point of 1720℃ and a dense hexagonal lattice structure when the temperature is lower than 882℃, which is called alpha titanium. Above 882℃ body – centered cubic character structure, known as beta titanium. Titanium alloys of different tissues are made by making use of the different characteristics of the two structures mentioned above and adding appropriate alloy elements to make the phase transition temperature and phase fraction content of titanium alloy change gradually. At room temperature, titanium alloys have three substrates, and titanium alloys fall into the following three categories: alpha alloys,(alpha + beta) alloys, and beta alloys. China is represented by TA, TC and TB respectively.
α titanium alloy
It is a single-phase alloy composed of alpha phase solid solution, which is alpha phase no matter in general temperature or in higher practical application temperature, with stable structure, better abrasion resistance than pure titanium and strong oxidation resistance. At the temperature of 500℃ ~ 600℃, it still maintains its strength and creep resistance, but cannot be strengthened by heat treatment, and its strength at room temperature is not high.
β titanium alloy
It is a single-phase alloy composed of beta phase solid solution, which has higher strength even without heat treatment. After quenching and aging, the alloy is further strengthened, and the strength can reach 1372 ~ 1666MPa at room temperature. But the thermal stability is poor, should not be used in high temperature.
α+β titanium alloy
It is dual-phase alloy, with good comprehensive properties, good structure stability, good toughness, plasticity and high temperature deformation properties, can be better for hot pressure processing, quenching, aging to strengthen the alloy. The intensity after heat treatment is about 50% ~ 100% higher than that after annealing. High temperature strength, can be in the temperature of 400℃ ~ 500℃ long-term work, its thermal stability is inferior to alpha titanium alloy.
The three most commonly used titanium alloys are alpha titanium and alpha + beta titanium; The machinability of a titanium alloy is the best, followed by a + beta titanium alloy, beta titanium alloy is the worst. A titanium alloy code for TA, beta titanium alloy code for TB, a + beta titanium alloy code for TC.
Titanium alloy can be divided into heat resistant alloy, high strength alloy, corrosion resistant alloy (ti-mo, ti-pd alloy, etc.), low temperature alloy and special function alloy (ti-fe hydrogen storage material and ti-ni memory alloy). The composition and properties of typical alloys are shown in the table.
Different phase composition and microstructure of titanium alloy can be obtained by adjusting heat treatment process. It is generally believed that fine equiaxial microstructure has good plasticity, thermal stability and fatigue strength. Acicular structure has high endurance strength, creep strength and fracture toughness. Isometric and acicular mixed tissues have better comprehensive properties.
Titanium alloy has high strength and low density, good mechanical properties, toughness and corrosion resistance. In addition, the process performance of titanium alloy is poor, cutting difficult, in hot processing, it is very easy to absorb hydrogen, oxygen, nitrogen, carbon and other impurities. There is poor wear resistance, complex production process. Industrial production of titanium began in 1948. The aviation industry needs to develop, so that the titanium industry with an average annual growth rate of about 8%. At present, the world's annual production of titanium alloy processing materials has reached more than 40,000 tons, titanium alloy brand nearly 30 kinds. The most widely used titanium alloys are ti-6al-4v (TC4), ti-5al-2.5sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).
Titanium alloy is mainly used to make aircraft engine compressor parts, followed by rocket, missile and high-speed aircraft structural parts. By the mid-1960s, titanium and its alloys were in common industrial use, making electrodes for electrolysis, condensers for power stations, heaters for oil refining and desalination, and pollution control devices. Titanium and its alloys have become corrosion resistant structural materials. It is also used to produce hydrogen storage materials and shape memory alloys.
China began research on titanium and titanium alloys in 1956. In the mid – 60 s, TB2 alloy was developed.
Titanium alloy is a new important structural material used in aerospace industry. Its specific gravity, strength and service temperature are between aluminum and steel. The f-84 fighter-bomber was first used by the United States in 1950 as a rear fuselage heat shield, wind shield, tail shield and other non-bearing components. In the 1960s, titanium alloy was used from the rear fuselage to the middle fuselage, partially replacing structural steel to make important load-bearing components such as frames, beams and flaps. The use of titanium alloys in military aircraft has increased rapidly, reaching 20 to 25 percent of the aircraft's structural weight. Since the 1970s, civil aircraft began to use a lot of titanium alloy, such as Boeing 747 passenger aircraft with titanium amount of more than 3640 kg. Aircraft with Mach Numbers less than 2.5 use titanium mainly to replace steel to reduce structural weight. Another example is the U.S. sr-71 high-altitude high-speed reconnaissance plane (flying Mach number is 3, flying at a height of 26212 meters). Titanium accounts for 93% of the weight of the aircraft structure and is known as "all-titanium" aircraft. When the thrust-weight ratio of aero-engine increases from 4 ~ 6 to 8 ~ 10, and the compressor outlet temperature increases from 200 ~ 300°C to 500 ~ 600°C, the low-pressure compressor disc and blade originally made of aluminum must be converted to titanium alloy, or titanium alloy is used to replace stainless steel to make the high-pressure compressor disc and blade, so as to reduce the structural weight. In the 1970s, the amount of titanium alloy used in aero engines generally accounted for 20% ~ 30% of the total weight of the structure. It was mainly used in the manufacture of compressor components, such as forged titanium fans, compressor disks and blades, cast titanium compressor casing, intermediate casing and bearing shell, etc.. The high specific strength, corrosion resistance and low temperature resistance of titanium alloy are mainly used to manufacture various pressure vessels, fuel storage tanks, fasteners, instrument straps, frames and rocket shells. Artificial earth satellites, lunar modules, manned spacecraft and space shuttle also use titanium alloy plate welding parts.
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